There are two distinct ways of using the liquid crystal properties for analyzing integrated circuits. These are:
(A) using the light scattering property of the liquid crystal (see reference 3 and 4), and
(B) the phase transition property of the liquid crystal (see reference 1 and 2).
This invention uses the phase transition property of the liquid crystal. Therefore, the discussion shall be limited to the hot spot detection method.
There are three kinds of liquid crystals: cholesteric, nematic and smectic. Both the cholesteric and nematic liquid crystal have been used for detecting hot spot (see reference 1 and 2). John Hiatt (see reference 1) reported that with a cross polarized light and a LC-127 cholesteric liquid crystal, he obtained a spatial resolution of ten to twenty microns. Also, the heating was not used, therefore the lowest detectable power of the hot spot is in the range of one hundred to two hundred milliwatts. E. M. Fleuren (see reference 2) reported the use of a N5 nematic liquid crystal phase to detect hot spots. The particular nematic liquid he used is called N5. He used a P.I.D. control and achieved a constant temperature of 0.1 degree celsius to a specified temperature. He could routinely detect a hot spot of 100 microwatts or more, with the P.I.D. control. However, by chance, if the liquid crystal's ambient temperature happens to be much less than 0.1 degree celsius (say a 0.005 degree celsius) below the liquid crystal phase transition temperature, he could detect a lower power hot spot. He managed to detect a hot spot of 3.6 microwatts once.
For a liquid crystal hot spot detection method, the lowest amount of integrated circuit energy required to induce a liquid crystal phase transition is proportional to the difference between the liquid crystal phase transition temperature and the liquid crystal film's temperature. Before this invention, the temperature control process is to keep the liquid crystal temperature constant. The disadvantage of the constant temperature method was that there is no instrument that could control a temperature infinitesimally close to a specified temperature, thus most low power hot spots cannot be detected. Also, prior to this invention, the heating mode was either conductive (see reference 2) or no heating at all (see reference 1). The liquid crystal film's temperature responds slowly to the conductive heat transfer, because a large poor heat conductor exists between the liquid crystal film and the conductive heating system. The same large poor heat conductor induces an uneven temperature profile on the liquid crystal film, thus reducing the sensitivity of the liquid crystal hot spot detection method.
This invention invented an infinitesimal temperature control method. This method enables the liquid crystal film's temperature to be brought to infinitesimally close below to the liquid crystal phase transition temperature. Therefore, a low power hot spot with only one or two microwatts could be routinely detected. The infinitesimal temperature control method can be achieved by many forms of heating means, such as, but not limited to, a conductive hot plate, a convective oven or a radiative heating light. All these heating means work with various degrees of effectiveness in the invented infinitesimal temperature control method. The key to the said infinitesimal temperature control method is that the heating means operates in a repeatedly turning on and turning off mode. It is during the turning on mode that the heating means will gradually heat up the liquid crystal film and will bring the film temperature infinitesimally close below to the liquid crystal phase transition temperature. To illustrate the said infinitesimal temperature control method, a detailed radiative heating light heating system is described in the SUMMARY OF THE INVENTION and the DETAILED DESCRIPTION OF THE INVENTION.
It should be emphasized that it is not the detailed design of the heating means that contributes to the primary success of bringing the liquid crystal film temperature infinitesimally close below to the liquid crystal phase transition temperature, rather it is the invented method of operating the heating means at a repeatedly turning on and turning off mode. The sensitivity of this invention was helped by using a pulsing input to the hot spot. The pulsing input induces a voltage induced blinking which can be mistaken for the hot spot induced blinking. The difficulty arising from the inability to differentiate between a voltage induced blinking and a hot spot induced blinking was solved by a differentiation method invented by this invention, that is, by observing the temperature responses of these two kinds of blinkings: The hot spot induced blinking increases in blinking spot size as the liquid crystal temperature rises, the voltage induced blinking remains in blinking spot size as the liquid crystal temperature rises.
Reference 1: John Hiatt, "A Method of Detecting Hot Spots on Semiconductors using Liquid Crystals." 19th Annual Proceedings of the IEEE Reliability Physics Symposium, 1981, Pg. 130-133. PA0 Reference 2: E. M. Fleuren, "A very sensitive, simple analysis technique using nematic liquid crystals," 21st Annual Proceedings of the IEEE Reliability Physics Symposium, 1983, Pg. 148-149. PA0 Reference 3: J. L. Fergason, "Liquid crystals in nondestructive testing," Applied Optics, Vol. 7, No. 9, Sept. 1968 Pg. 1729-1737. PA0 Reference 4: G. D. Dixon, "Cholesteric liquid crystal in nondestructive testing," Material Evaluation, June 1977, Pg. 51-55.